01 - Preheat a large skillet or griddle over medium heat.
02 - Spread butter evenly on one side of each bread slice. Place two bread slices, butter side down, on a clean surface.
03 - Optional: Spread Dijon mustard on the unbuttered side of the bread. Layer half the cheddar cheese onto each slice.
04 - Top evenly with pickle slices, then a generous handful of popcorn. Sprinkle with garlic powder and black pepper if desired. Add remaining cheese on top.
05 - Place the remaining bread slices on top, butter side up, to form two sandwiches.
06 - Carefully transfer sandwiches to the skillet. Cook 3-4 minutes per side, pressing gently, until bread is golden and cheese is melted. Adjust heat if necessary to prevent over-browning.
07 - Remove sandwiches from the skillet, allow them to rest for 1 minute, then slice and serve immediately.